The present invention relates to a temperature measuring apparatus using temperature detecting means, such as a thermistor, a temperature measuring resistor, or a thermocouple, for measuring temperatures by converting temperature changes into a resistance variation or a thermal electromotive force and measuring the resistance value or the thermal electromotive force.
In a prior-art temperature measuring apparatus, temperature is measured by using a thermistor or a temperature measuring resistor for converting a temperature change into a resistance variation or a thermocouple for outputting a thermal electromotive force. However, since these elements produce non-linear resistance variation and thermal electromotive force with respect to the temperature change, it is necessary to include means which provide linearizing functions so as to increase the measuring accuracy, especially when used to measure a wide temperature range. FIG. 1 shows a temperature-resistance change of a typical thermistor. In FIG. 1, the abscissa axis represents temperatures T and the ordinate axis represents values R. Since the temperature cannot be directly read out from the resistance value if the non-linear varying characteristic of the curve shown in FIG. 1 is used, means providing a linearizing function, such as shown in FIG. 2, is incorporated. In FIG. 2, reference numeral 1a denotes temperature detecting means, numeral 1 denotes a thermistor connected in parallel with a resistor 2, further connected in series with a resistor 3, and connected between a power source P and ground E of the circuit. Thus, a voltage V between the ground E and the terminal t is produced as an output in accordance with the resistance of the thermistor 1.
FIG. 3 illustrates a curve representing a relationship between the output voltage V and the temperature. As shown, the abscissa axis represents temperatures T and the ordinate axis represents voltages V. It is noted that, as the temperature rises, the output voltage drops substantially linearly. When the resistors 2 and 3 are adequately selected, the curve in this graph can be formed to be substantially linear. However, if the measuring range is wide or an accurate temperature measurement is necessary, this linearization is insufficient.
In the temperature detecting means of the above-described temperature measuring apparatus, it becomes necessary to provide a further linear characteristic if the measuring temperature range is widened or an accurate temperature measurement is necessary, but, to this end, a linearizing circuit called a "linearizer" is necessary. However, such a circuit must use a number of accurate resistors, variable resistors, or operational amplifiers, its regulation is complicated, and it is expensive to provide accurate parts.